Modulators are extensively used in consumer products such as VCRs, satellite receivers, camcorders or similar video systems to couple received audio and video signals to a display unit such as a TV receiver. The basic function is to take inputs of base-band composite video and audio, and to modulate a RF carrier such that a TV receiver can tune to this signal.
There are a number of different video standards used around the world. For example, in Europe there exists PAL B/G, PAL I, SECAM L and SECAM B/G. Each standard has an unique specification which must be met by the video system and hence the modulator of the system. This means that a PAL VCR requires a modulator having different operating parameters to that of a modulator of a SECAM VCR. These parameters include O/P channel number, modulation depth, audio (sound) carrier frequency. A manufacturer supplying the European video market must therefore make different versions of modulators for the different video standards: each version designed to serve a specific video system.
Multi-standard modulators have now been designed in order to avoid the need to produce many different versions of modulators. These multi-standard modulators can operate in two or more different systems when fitted with appropriate external components. These modulators employ either fixed, selected, components or potentiometer/trimmer controls for adjusting parameters in order to meet a certain standard's specification.
In order to provide multi-standard capability, the known modulator circuits use a significantly large mount of extra hardware and external switching. For example, the audio carrier frequency of a modulator is determined by the values in an external L-C tank circuit. A multi-standard modulator must provide three or more audio carriers and thus requires at least three of such L-C tanks. In addition to the disadvantages associated with extra hardware, each of the tank circuits require factory set up and external switching to the appropriate standard. This is time consuming which increases production costs.
Trimming adjustments of modulator parameters, such as audio and video depth, picture/audio carrier ratio, are factory set using either manual potentiometers or SOT (Select-On-Test) components. Such manual adjustments are time consuming and subject to operator error. Furthermore, with multi-standard modulators, it is often necessary to further vary these parameters in order to meet the requirements of a specific standard, in which case additional externally controlled switching is required.
In summary, the known multi-standard modulators avoid the need to produce different types of modulators for the different standards and thereby increase the flexibility for the manufacturer. However, the known multi-standard modulators are more complex circuits requiring additional components and manual adjustments which increases the size and production costs for such modulators.